For many years it has been known that the safety ground in AC outlets in important, this ground connection will shunt current which can appear on the enclosure of the tool or appliance, due to a defect in insulation, harmlessly to ground. If the enclosure were not properly grounded and the hot wiring of the power tool were to short to the case, the user could suffer injury or death if he or she were standing on a surface which had a path to earth ground or were touching a surface which was connected to earth ground. If the tool being used were to develop such a fault and the outlet wiring was proper, a large amount of current would flow from the HOT wire of the AC outlet through the tool and back through the safety ground of the AC outlet, thus exceeding the current rating of the circuit breaker or fuse which protects the wiring and tripping the breaker or blowing the fuse. It now becomes apparent that the total resistance of the HOT and SAFETY GROUND must have a value low enough to allow a current exceeding the rating of the circuit breaker or fuse to flow thus insuring that the power to the circuit will be interrupted due to such a fault.
Most 120 VAC circuit breakers have a rating of 20 amps. Using Ohms Law only, we find that the total resistance of the wiring from the main panel to the AC outlet and the connection from the AC outlet to earth ground must not exceed 6 ohms. If this total resistance is greater than 6 ohms, the breaker may not trip during a fault and a dangerous condition in terms of fire hazard or shock will result. The potential for poor electrical connections to earth ground are the most prevalent since these connections are usually made through the metallic conduit which carries the wiring. This conduit typically has many couplings and connections through junction boxes between the AC outlets and the connection to earth ground which usually terminates at the power panel. In addition to faulty assembly of these ground returns, the connections over time may loosen or corrode causing the resistance to increase.
As shown in the example above, the total resistance of the circuit must not be greater than 6 ohms. The most popular outlet tester on the market uses neon bulbs to indicate AC outlet miswires, these lamps requiring only 1 or 2 milliamperes to light or turn on. A faulty ground connection having a resistance as high as 120,000 ohms will light the lamp in this type of tester and the user will assume the outlet is safe to use when it is not.
It becomes apparent that more than 1 or 2 milliamperes is required to properly exercise these wiring connections, but a problem arises with ground fault interrupter outlets. These outlets will trip if more than 3 milliamperes of current flow from the HOT WIRE and through the ground return of the outlet. Thus, a tester which causes more than 3 milliamperes to flow through these wires during testing is not usable since the interrupter would trip every time it was tested. Applicant has found that the solution to this problem is to draw high current short duration pulses, currents exceeding 20 amps may be drawn from the HOT WIRE to EARTH GROUND through the outlet without tripping the interrupter, if the duration is short enough. A total duration of 3 to 5 microseconds with peak currents of 20 amps or more will not trip these interrupters and the resistance of the wiring can now be determined by detecting the amplitude of the current pulses. If the total resistance of the circuit is lower than a predetermined limit, the peak current will be high enough to trigger a light indicating the outlet is safe to use.
In view of that set forth above and other known deficiencies of the prior art for testing not only that an AC outlet is properly wired but also to determine any defect in the connection of the hot, neutral and ground wire connections, and further to detect a defective ground, the present invention provides the following advantages.
It is an object of the invention to provide a tester unit for detecting and indicating the occurrence of proper wiring of a tested AC outlet, a defective ground, an open Neutral, reversed Hot and Neutral connections, a connection wherein "Hot on Neutral with open Neutral" occurs, and an open Hot connection.
It is another object of the invention to assure the user that the AC outlet has been wired properly and the total resistance of the Hot wire and the Ground return is below 6 ohms.
Various other objects and advantages of the present invention will be made known upon reading and understanding the detailed description of the invention which follows.